The invention relates to the preparation of vinyl aromatic-allylic alcohol copolymers. In particular, the invention relates to the preparation of copolymers having low molecular weights and low hydroxyl functionality.
Styrene-allyl alcohol (SM) copolymers are known. U.S. Pat. Nos. 2,588,890 and 2,630,430 teach copolymerzing styrene with allyl alcohol in the presence of oxygen as a catalyst. The SM copolymer thus prepared has high gel content and inferior properties.
U.S. Pat. Nos. 2,894,938 and 2,900,359 teach copolymerizing styrene with allyl alcohol using organic peroxides as initiators in the absence of oxygen. The polymerization is conducted in a bulk process by charging allyl alcohol, styrene and an initiator into a reactor and heating the reaction mixture to a polymerization temperature (180xc2x0 C. to 300xc2x0 C.). The SAA copolymer has a low gel content and a functionality greater than 4.
U.S. Pat. No. 2,940,946 teaches a semi-batch process for making SAA copolymers. The process is conducted by initially charging allyl alcohol, an initiator, and a portion of styrene into a reactor, and adding the remaining styrene gradually into the reaction mixture during the polymerization. The copolymer has low color and improved thermal stability.
U.S. Pat. Nos. 5,444,141 and 5,886,114 teach the preparation of SAA copolymers by initially charging allyl alcohol, a portion of styrene and a portion of an initiator into a reactor and gradually adding the remaining styrene and initiator into the reaction mixture during the polymerization. The process gives substantially higher yields compared with the process disclosed in U.S. Pat. No. 2,940,946.
U.S. Pat. No. 6,103,840 teaches an improved process over that of U.S. Pat. No. 5,444,141. The process is conducted by increasing the reaction temperature during the addition of styrene and initiator. The process gives even higher yields of SAA copolymers.
All of the known processes are conducted without the use of a solvent. These processes invariably produce SAA copolymers having hydroxyl functionality (i.e., the number of hydroxyl groups per polymer chain) greater than 4 without using oxygen as a catalyst. When oxygen is used, the copolymers have lower functionality, but they also have high gel content and other inferior properties.
SAA-100 and SAA-101 have been commercially produced for several decades. They have 70/30 and 60/40 molar ratios of recurring units of styrene to allyl alcohol, respectively. Although these SAA copolymers differ in the hydroxyl content or hydroxyl number (OH#, SAA-100: 200 mg KOH/g; SAA-101: 255 mg KOH/g), they have essentially the same hydroxyl functionality (about 5). High hydroxyl functionality of the copolymers gives SAA-based coatings high crosslinking density. However, high functionality also limits the use of the copolymers in many areas, such as in the synthesis of polyester resins where SAA can cause gel formation.
Attempts to reduce the hydroxyl functionality of SAA copolymers have been made. One might try to reduce hydroxyl functionality by reducing the concentration of allyl alcohol during polymerization. This approach fails, however, because the current SAA polymerization process inherently gives higher molecular weight polymers when less allyl alcohol is used. For example, we have recently introduced SAA-103, which has only about 20 mole % of the recurring units of allyl alcohol (hydroxyl number: 125 mg KOH/g). Surprisingly, SAA-103 has even higher hydroxyl functionality (about 7) than SAA-100 or SAA-101. While SAA-103 has a lower concentration of hydroxyl groups, it also has much longer chains, so the number of hydroxyl groups per polymer chain is actually higher than SAA-100 or SAA-101.
A new process for producing SAA copolymers is needed. Ideally, the process would produce SAA copolymers having low hydroxyl functionality and low molecular weights.
The invention is a process for making copolymers of a vinyl aromatic monomer and an allylic alcohol. The process is performed in the absence of oxygen. The process begins with charging a reactor with an allylic alcohol, 0-50% of the total amount to be used of a vinyl aromatic monomer, 0-100% of the total amount to be used of a free-radical initiator and an organic solvent in an amount greater than or equal to about 10% by weight of the total amount of the vinyl aromatic monomer. The reaction mixture is then heated at a temperature within the range of about 100xc2x0 C. to about 185xc2x0 C. The remaining vinyl aromatic monomer and initiator are added to the reaction mixture at a decreasing rate during the polymerization.
In the process of the invention, a reactor is initially charged with an allylic alcohol. Allylic alcohols useful in the process preferably have the general structure: 
in which R is selected from hydrogen, a C1-C10 alkyl group, or a C6-C10 aryl group. Examples of suitable allylic alcohols are allyl alcohol, methallyl alcohol, and 2-ethyl-2-propen-1-ol. Mixtures of allylic alcohols can be used. Allyl alcohol is preferred because it is commercially available and inexpensive.
The amount of allylic alcohol to be used is determined by many factors. They include the desired hydroxyl number of the copolymer, the reaction temperature, the amount of vinyl aromatic monomer to be used, the amount of initiator to be used, and the manner of the addition of the vinyl aromatic monomer and the initiator. Determining how much allylic alcohol to be used is further complicated by the low reactivity of allylic alcohols. Allylic monomers have much lower reactivity than vinyl aromatic monomers. The great disparity in the monomeric reactivities requires a large excess of allylic alcohols in the reaction mixture to achieve an adequate incorporation of allylic alcohols in the copolymer. In general, more than 25% of excess allylic alcohol is needed. The unreacted allylic alcohol is removed from the polymer after polymerization and is reused.
The reactor is initially charged with 0-50% of the total amount to be used of a vinyl aromatic monomer. Suitable vinyl aromatic monomers preferably have a xe2x80x94CRxe2x80x2xe2x95x90CH2 group connected to an aromatic group. Rxe2x80x2 is hydrogen or a C1 to C10 alkyl group. Examples of suitable vinyl aromatic monomers are styrene, xcex1-methylstyrene, xcfx81-methylstyrene, xcfx81-t-butylstyrene, 9-vinylanthracene, 2-vinylnaphthalene, and the like, and mixtures thereof. Styrene is particularly preferred. The total amount of vinyl aromatic monomer to be used is determined mainly by the desired copolymer composition. Vinyl aromatic monomers polymerize essentially completely.
The remaining vinyl aromatic monomer is gradually added, at a decreasing rate, into the reactor during the course of polymerization. At least 50% of the vinyl aromatic monomer is added to the reaction mixture gradually during the polymerization. Preferably, the ratio of the vinyl aromatic monomer to allylic alcohol is kept essentially constant so that the copolymer produced has a relatively uniform composition.
The process comprises initially charging the reactor with 0-100% of the total amount of a free-radical initiator. Suitable free-radical initiators include peroxides, hydroperoxides, peresters, azo compounds, and many others known to the polymer industry. Examples of suitable free-radical initiators are hydrogen peroxide, di-t-butyl peroxide, t-butyl hydroperoxide, cumene hydroperoxide, t-butyl perbenzoate, 2,2xe2x80x2-azobisisobutyronitrile, and the like, and mixtures thereof. The total amount of the initiator to be used is determined by many factors, but mainly by the desired polymerization rate. When more initiator is used, faster polymerization is achieved. Surprisingly, the molecular weight of the copolymer does not vary significantly with the change of the initiator concentration.
It is preferred to add at least 50% of the total amount to be used of the initiator to the reactor gradually during the course of the polymerization. It is also desirable to keep the ratio the free-radical initiator to the vinyl aromatic monomer essentially constant so that the copolymer produced has narrow molecular weight distribution.
The process is performed in the absence of oxygen. The process comprises charging the reactor with an organic solvent in an amount greater than or equal to about 10% by weight of the total amount of the vinyl aromatic monomer. Preferably, the solvent is charged initially into the reactor. Alternatively, the solvent is charged partly into the reactor initially, and the remaining is added during the polymerization. Preferably, the solvent is used in an amount greater than about 20 wt % of the total amount of the vinyl aromatic monomer. More preferably, the amount of solvent is greater than about 50 wt % of the total amount of the vinyl aromatic monomer. Suitable solvents are those in which the copolymer is soluble under the polymerization conditions. They include C4-C18 linear or cyclic aliphatic hydrocarbons, C8-C18 aromatic hydrocarbons, esters, ethers, ketones, alcohols, glycol ethers, and the like, and mixtures thereof. Examples of suitable solvents are toluene, xylenes, cyclohexane, methyl amyl ketone, butyl acetate, and propylene glycol methyl ether acetate. Toluene and xylenes are preferred. We surprisingly found that the use of an organic solvent enables the preparation of SAA copolymers having both low molecular weight and low hydroxyl functionality.
The polymerization is conducted at a temperature within the range of about 100xc2x0 C. to about 185xc2x0 C. Increasing temperature reduces the disparity of the monomeric reactivities between the vinyl aromatic monomer and the allylic alcohol, and thus enhances the incorporation of the allylic monomer into the copolymer. However, increasing temperature also induces high pressure, which increases the risk of the operation. When allyl alcohol is used, the polymerization is preferably performed under relatively low pressure because allyl alcohol is highly toxic. Preferably, the polymerization is conducted at a temperature from about 125xc2x0 C. to about 165xc2x0 C.
The invention includes copolymers made by the process. The copolymers differ from those known in the art in that they have lower hydroxyl functionality. The copolymers have an average hydroxyl functionality less than about 4. Preferably, the copolymers have average hydroxyl functionality from about 1.5 to about 2.5. The copolymers made by the process of the invention not only have low hydroxyl functionality but also have essentially no gel content. By xe2x80x9cgel,xe2x80x9d we mean that the polymer is crosslinked during the polymerization and becomes partially insoluble in the solvent. When a polymer has no gel content, its solution is clear. In addition, the copolymers have much lower solution viscosity than the existing products.
Preferably, the copolymers have a number average molecular weight from about 1,000 to about 3,000 and a molecular weight distribution from about 1.5 to about 5.5. The copolymers preferably have a hydroxyl number from about 30 mg KOH/g to about 150 mg KOH/g. The preferred copolymer made by the process is a styrene-allyl alcohol copolymer.